HR: 17:00h
AN: T14B-05    [Abstracts]
TI: Grain scale processes during cataclasis and healing in granitoid fault rocks: results from experiments and nature
AU: Keulen, N
EM: nynke.keulen@unibas.ch
AF: Geological Institute, Basel University Bernoullistr. 32, Basel, CH-4056 Switzerland
AU: * Stunitz, H
EM: holger.stuenitz@unibas.ch
AF: Geological Institute, Basel University Bernoullistr. 32, Basel, CH-4056 Switzerland
AU: Heilbronner, R
EM: renee.heilbronner@unibas.ch
AF: Geological Institute, Basel University Bernoullistr. 32, Basel, CH-4056 Switzerland
AB: Earthquakes lead to the formation of fault rocks with a wide range of grain sizes and shapes. Continued displacement after the main stress release modifies the microstructure. With time, the resulting fault gouge may heal again acquiring a different grain size distribution and a reduced permeability compared to the unconsolidated fault gouge. In addition to (static) healing diffusion creep may occur in the very fine grained aggregates formed during cataclasis, even at relatively low temperatures. By performing deformation experiments we attempt to simulate the static and dynamic processes that govern the strength of fault rocks during the seismic and aseismic interval. Experiments: Coaxial deformation experiments were carried out on isotropic Verzasca gneiss using a Grigg's deformation apparatus at 300-500 °C, 500-1000 MPa, strain rates of 10^-4 s^-1 to 10^-7 s^-1 and 0.2 %wt H₂O added. 4 types of experiments were performed: 1-Deformation-only: Samples were fractured in coaxial compression and quenched immediately. 2-Healing: After fracturing the samples were kept at hydrostatic conditions for 4 to 14 days at 300 or 500 °C. 3-Strain rate tests: Samples were deformed at variable strain rates to study the interaction between deformation and healing for 4 to 14 days. 4-Repeated fracturing: A cycle of fracturing - healing - fracturing was preformed to study the strength evolution of the fault gouge. Micorstructure analysis: The experimentally deformed granitoids were compared to natural fault rock samples originating from the Nojima Fault Zone (Kobe Earthquake, 1995) and from the Alps (deformed in the Tertiary). SEM images with different magnifications and digital image analysis methods were used for the analysis of the microstructures and the grain size distribution. Special shape descriptors (for convexity/concavity and angularity) are calculated which can be used to quantify the evolution of the fault rock from cracked minerals to fault gouge to healed rock. The grain size distribution describes the evolution from fresh fault gouge to healed fault gouge. Cracked fragments and mature gouges can be differentiated on the basis of the slope D of the power-law fit of the grain size distribution: D of the cracked material ~ 1.5, D of the gouge ~ 2.0, D below the grinding limit (1-3 μm) ~ 1.0. In the case of the gouge, the fractal nature of the grain size distribution is questioned. D (slope) mapping was used to visualize the spatial distribution and connectivity of gouge and cracked material. Fresh fault gouge shows very small grains (down to 30 nm). The smallest grains are consumed in the healing process, very few grains smaller than 1 μm are present after healing. Healing experiments: The healing of the fault gouge is enhanced by increasing healing time, temperature and stress. Samples healed during slow stress relaxation or low strain rates (≤ 10^-7 s^-1) appear more efficiently healed than samples healed under hydrostatic conditions. Solution transfer processes are probably operating during slow deformation in our experiments. These results are applicable to post-seismic creep after the main stress release in earthquake zones. Healing and continued deformation occur simultaneously.
DE: 8159 Rheology: crust and lithosphere (8031)
SC: Tectonophysics [T]
MN: Fall Meeting 2005